Electro-optic devices are frequently used in fiber-optic telecommunication systems to manipulate optical signals. In general, these electro-optic devices include at least one optical waveguide formed from and/or in an electro-optic material. When an electric field is generated in the electro-optic material, the refractive index of the optical waveguide(s) change and the optical signal propagating there through can be altered. Some examples of common electro-optic devices used in telecommunication systems include optical modulators, optical switches, optical couplers, etc.
One example of a particularly successful electro-optic device is the Mach-Zehnder (MZ) optical modulator. Referring to FIG. 1a, there is shown an embodiment of a Mach-Zehnder optical modulator having an optical waveguide 20 formed in an electro-optic substrate 10. The optical waveguide 20 includes a first Y-branch 22, a first interferometer arm 24, a second interferometer arm 26, and a second Y-branch 28. An electrode structure (not shown in FIG. 1a) is provided near/adjacent the optical waveguide 20 for generating an electric field in one or both interferometer arms 24/26. For example according to one well known configuration, the electrode structure includes a signal electrode (also often referred to as a hot electrode) and two ground electrodes, which are configured to generate oppositely oriented electric fields in the first 24 and second 26 interferometer arms. Conventionally, the electrode structure is formed from a highly conductive metal such as gold (Au). The exact position and design of the electrodes relative to the optical waveguide 20 is generally dependent on the substrate. For example, if the substrate is formed from X-cut lithium niobate (LiNbO3), the signal electrode 40 is typically positioned on top of the substrate substantially between the interferometer arms 24/26, while the ground electrodes 42/44 are positioned on top of the substrate outside of the interferometer arms 24/26 (e.g., as illustrated in FIG. 1b). In contrast, if the substrate is formed from Z-cut LiNbO3, the signal electrode 40 is typically positioned substantially above one interferometer arm 26, while the ground electrode 42 is positioned substantially above the other interferometer arm 24 (e.g., as illustrated in FIG. 1c). In each case, the ground electrodes 42,44 are typically connected to ground, while the signal electrode 40 is connected to a high-frequency power source.
Referring to FIG. 1d there is shown an embodiment of a Z-cut LiNbO3 modulator 100, wherein the signal electrode 140 is connected to a high-frequency power source 145 at one end and to a terminal resistor at the other end, such that it functions as a traveling-wave electrode. In operation, an optical signal is input into the left side of the device 100 where it is transmitted through the optical waveguide 120 until it is split at the first Y-branch 122, and then propagates equally along the two isolated paths corresponding to the two interferometer arms 124/126. Simultaneously, an RF data signal from the high-frequency power source 145 is transmitted through an RF transmission line 147 (e.g., a co-axial cable) to the signal electrode 140, which functions as a microwave transmission line. As the modulation voltage is applied between the signal electrode 140 and the ground electrodes 142 and 144, an electric field is generated in the underlying electro-optic substrate 110. As illustrated in FIG. 1e, the vertical electric field lines in the first 124 and second 126 interferometer arms are oppositely oriented such the light propagating in each of the arms is complementarily phase shifted relative to one another in a push-pull fashion. In accordance with the electro-optic effect, the electric field changes the refractive index within the interferometer arms such that the input optical signal experiences constructive or destructive interference at the second Y-branch 128. This interference produces an amplitude modulated optical signal that is output the right side of the device, wherein the modulation corresponds to the original RF data signal.
Notably, since the Z-axis of a LiNbO3 crystal has the highest electro-optic coefficient, Z-cut LiNbO3 modulators exhibit a relatively high modulation efficiency. Unfortunately, Z-cut LiNbO3 modulators are also known to suffer more from charge build up problems, which for example, may lead to temperature induced bias drift and/or DC drift.
Temperature induced bias drift refers to when the operating (bias) point of the modulator shifts with changes in temperature. In LiNbO3, temperature induced bias drift typically arises from the pyroelectric effect, which creates mobile charge when temperature fluctuations occur in the substrate. The mobile charge can generate strong electric fields that can change the operating (bias) point of the electro-optic modulator. In addition, since the electric fields induced by the pyroelectric effect in Z-cut LiNbO3 are predominantly normal to the substrate, the mobile charge moves toward the surface of the substrate, where the electrodes 140, 142, 144 are located. Accordingly, a bleed layer 160 is typically required near the surface of Z-cut LiNbO3 to dissipate accumulated electric charge. Optionally, additional bleed layers (not shown) are used to dissipate charge at the sides or bottom of the substrate. In general, the bleed layer 160 will be formed from a semiconductive material so that the highly conductive electrodes 140/142/144 are prevented from shorting out.
DC drift refers to when the operating (bias) point of the modulator shifts as a low frequency or DC voltage is applied to the modulator for extended periods of time. In general, low frequency or DC voltages are required to control the operating (bias) point of the modulator (i.e., the point about which the swing of the modulated RF signal is accomplished). For example in the embodiment described with reference to FIG. 1d, the RF data signal corresponds to a modulation signal that includes an RF component superimposed on a DC or low frequency component.
DC drift, also termed bias drift, is particularly problematic when the modulator includes a buffer layer 150 disposed between the substrate 110 and the signal electrode 140. If the buffer layer 150 has little conductivity relative to the substrate 110, mobile charge within the substrate, which may be in the form of electrons, holes, or ions, counteracts the effect of the applied voltage, establishing a positive DC drift. In addition, impurities in the buffer layer 150, which is typically formed from a dielectric material such as silicon dioxide (SiO2), are believed to form additional mobile charge, which either counteract the effect of the applied voltage, establishing a positive DC drift, or enhance the applied bias voltage, establishing a negative DC drift. The former is more common for undoped SiO2. The end result of the mobile charge in the buffer and substrate is that the bias voltage required to operate the electro-optic modulator increases over time.
The purpose of the buffer layer 150 is two-fold. First, the buffer layer 150 is used to prevent optical absorption of the optical signal by the overlying electrodes 140/142. Notably, this is more important for Z-cut embodiments, wherein the electrodes 140/142 lie directly over the interferometer arms 126/124. Secondly, the buffer layer 150 is used to speed up the propagation of the RF modulation signal so that the optical wave and the microwave propagate with equal phase velocities, thus increasing the interaction length, and as a result, increasing modulation bandwidth and/or efficiency at high frequencies.
Various solutions to prevent dc drift have been proposed. For example, in X-cut LiNbO3 modulators it has been proposed to provide a separate low-frequency bias electrode structure 270, optically in series with an RF electrode structure 240, as illustrated in FIG. 2. A buffer layer 250 is provided below the RF electrode structure 240, to provide velocity matching, but is eliminated below the bias electrode structure 270, to reduce DC drift. Conveniently, since the bias electrode structure 270 is deposited directly on the substrate, the required drive (bias) voltage is significantly reduced. Unfortunately, in order to accommodate both electrode structures, the length of the modulator is significantly increased. In addition, this design is not ideal for Z-cut LiNbO3, wherein the waveguides are located directly below the bias electrodes, because the highly conductive bias electrode material (e.g., Au) may introduce significant optical loss.
In Z-cut LiNbO3 modulators, dc drift has been reduced by modifying the buffer layer. For example in U.S. Pat. Nos. 5,404,412 and 5,680,497, the effect of the buffer layer charging in optical modulators is reduced by doping the buffer layer, causing it to be more conductive. The added conductivity in essence shorts out the buffer layer, preventing the buffer layer from charging up and stealing all of the applied voltage from the waveguides. Accordingly, a DC or slowly varying voltage applied to the signal electrode is able to control the bias point of the modulator over time. Unfortunately, it can be difficult to quantitatively control the introduction of the doping elements with a high reproducibility. Furthermore, water may be absorbed by the conductive buffer layer, changing its properties. In addition, the required drive (bias) voltage may be relatively high because the generated electric field must pass through the conductive buffer layer (e.g., which may be quite thick for Z-cut configurations). In US Patent Application Publication No. 2003/0133638 DC drift is reduced by implanting a SiO2 buffer layer with fluorine ions. The negative fluorine ions (F−) are believed to react with positive ions, such as lithium (Li+) from the substrate, to form stable compounds such as LiF. The reduction in the number of mobile Li+ ions then results in a reduction in DC drift. Again, the required drive (bias) voltage may be relatively high because the generated electric field must pass through the ion-implanted buffer layer (e.g., which may be quite thick for Z-cut configurations).
In US Patent Application Publication No. 2006/0023288 and U.S. Pat. No. 7,127,128, DC drift is reduced by providing a separate low-frequency bias electrode structure substantially aligned with an overlying RF electrode structure. For example, consider the prior art X-cut embodiment illustrated in FIGS. 3a and 3b, wherein a dielectric buffer layer 350 is provided below the RF electrode structure 340/342/344, to provide velocity matching, but is eliminated below the bias electrode structure 370/372/374, to reduce DC drift. Advantageously, this configuration provides a relatively short modulator (i.e., since the bias and RF electrode structures are stacked) with a relatively low drive voltage (i.e., since the bias electrode structure is deposited directly on the substrate 310). Further advantageously, the bias electrode structure 370/372/374 is fabricated from a material having a high resistivity, which is conductive at low frequencies and functions as a dielectric at high-frequencies. Accordingly, the bias electrode structure can be deposited on the substrate without introducing significant loss.
US Patent Application Publication No. 2006/0023288 also describes numerous low bias drift embodiments for Z-cut LiNbO3 modulators. Referring to FIGS. 4a and 4b, the Z-cut embodiments typically include two bias signal electrodes, each of which is split into two separate elongated segments. More specifically, each segment of each split bias electrode 470/476 is shifted laterally to an opposite side of the corresponding waveguide segment 426/424. Again, a dielectric buffer layer 450 is provided below the bleed layer 460 and RF electrode structure 440, 442, 444, to provide velocity matching, but is eliminated below the bias electrode structure 470, 472, 474, 476, to reduce DC drift. Advantageously, this configuration provides a relatively short modulator (i.e., since the bias and RF electrode structures are stacked) with a relatively low drive voltage (i.e., since the bias electrode structure is deposited directing on the substrate 410). Further advantageously, since the bias signal electrodes 470, 476 are split, and are not disposed directly over the interferometer arms 426, 424, respectively, optical loss is reduced.
Yet another advantage of many of the embodiments described in US Patent Application Publication No. 2006/0023288 is improved humidity tolerance. As is well known in the art, the presence of high magnitude electric fields and high humidity often results in corrosion of electro-optic devices. For example, when a metal adhesion layer (e.g., Ti, Ti/W, Cr, etc) is used to promote adhesion between an RF electrode (e.g., Au) and an electro-optic substrate (e.g., LiNbO3), any moisture in direct contact with the multi-layer structure will serve as an electrolyte that induces galvanic corrosion. Galvanic corrosion, which results from the difference in electrochemical potentials of dissimilar metals, can create a conductive deposit between the surface RF electrodes, which causes current leakage, short circuit, or peeling of the RF electrodes. Various schemes have been proposed to obviate galvanic corrosion, and thus reduce the need for a hermetic package. For example, in U.S. Pat. No. 6,867,134 the adhesion layer is eliminated, whereas in US Patent Application Publication No. 2003/0062551 the adhesion layer is encapsulated. Alternatively, the adhesion layer can be made of a thin metal, such as nickel, which has a work function similar to gold. While these methods do suppress galvanic corrosion, electro-migration corrosion can still occur. Electro-migration corrosion occurs when a large DC voltage is applied across closely-spaced electrodes (e.g., gold RF electrodes) in the presence of water or a high humidity level. Similar to galvanic corrosion, electro-migration corrosion negatively impacts the performance and reduces the service life of electro-optic devices. As a result, electro-optic devices are often coated as shown in U.S. Pat. No. 6,560,377 and/or sealed in hermetic packages. However, the coatings negatively impact the RF properties of the RF electrode, and hermetic packaging adds cost to the modulator.
In US Patent Application Publication No. 2006/0023288 humidity tolerance is increased in various ways. For example in some embodiments, the large DC voltage is applied to bias electrodes that are disposed beneath a buffer layer, whereas in other embodiments the large DC voltage is applied to bias electrodes that are disposed below the substrate. Since these buried bias electrodes are protected from humidity, electro-migration corrosion of the buried bias electrodes is reduced. Moreover, if the buried bias electrodes are DC isolated from the RF electrodes, then electro-migration corrosion of the RF electrodes is also minimized. Furthermore, if the adhesion layer is eliminated, encapsulated, and/or formed of a material with a work function similar to that used to form the RF electrode, then both galvanic and electro-migration corrosion mechanisms are eliminated, enabling low cost non-hermetic packaging of the modulator.
In addition, improved humidity tolerance is also provided by fabricating the bias electrodes from a high resistivity material (e.g., a material having an electrical resistivity substantially higher than that of the RF electrodes, but substantially lower than the substrate). Notably, these high resistivity bias electrodes have been found to be significantly more robust than prior art high-conductivity bias electrodes (e.g., fabricated from gold). Unfortunately, selection of the high resistivity material is generally accompanied with a compromise in at least one of RF transparency, optical transparency, and series resistance. For example, while indium oxide (In2O3) is reasonably optical transparent, with a resistivity of approximately 1 ohm-cm (Ω-cm) @25° C. there may be some RF signal loss. In contrast, while TaSiN having a resistivity in the range of about ˜104 to 106 Ω-cm @25° C. is reasonably transparent to the RF signal, it may introduce some optical loss if deposited directly above an optical waveguide.
In U.S. Pat. No. 6,853,757 Seino teaches using a transparent conductive film underneath a highly conductive metal electrode to apply a voltage to an electro-optic substrate. The highly conductive metal electrode (e.g., gold) is shifted laterally with respect to the center of the waveguide to minimize optical loss. However, since the transparent conductive film (e.g., indium tin oxide (ITO)) is intended to carry both high and low frequency signals from the highly conductive electrode to the waveguide, the electro-optic device is not humidity tolerant and must be coated as shown in U.S. Pat. No. 6,560,377 and/or sealed in a hermetic package.